The characteristics of the IEEE 802.15.4 physical and medium access layers make such a protocol a suitable candidate to support communication between sensors and actuators in industrial environments. As industrial networks may comprise a large number of sensors and actuators and the delay increases with the increased number of nodes, a possible solution to keep the delay small is the use of multiple radio channels to implement different small low-latency communication cells. Although in IEEE 802.15.4 the radio channels do not overlap, recent literature showed that some interference may actually occur. This paper provides a better understanding of cross-channel interference in co-located IEEE 802.15.4 industrial networks and proposes a general methodology for the assessment of IEEE 802.15.4 performance under different cross-channel interference conditions. This methodology allows a network designer to perform on-site but accurate assessments and can be easily deployed in real industrial environments to perform measurements directly in the environment-under-test. A case study based on COTS IEEE 802.15.4 devices is presented to show how to apply our methodology to a real scenario and to discuss the results obtained with one or multiple interferers and varying some MAC level parameters.